(i) Field of the Invention
The present invention relates to a low-NOx gas burner which can be used in an absorption type water cooler/heater, an absorption type refrigerator, a vapor boiler and a hot water boiler and in which the production of nitrogen oxides (hereinafter abbreviated to "NOx") can be inhibited.
(ii) Description of the Prior Art
In recent years, it has been considered that air pollution increases gradually due to NOx and the like in exhaust gases discharged from factories and offices, and measures for the control of the air pollution are strongly desired.
NOx produced by the combustion of fuels can be classified into thermal Nox formed by the reaction of nitrogen and oxygen contained in air for combustion in a high-temperature state and fuel NOx formed by the oxidation of various nitrogen compounds contained in the fuel during combustion. In most of the cases where a gaseous fuel is used, the thermal NOx is formed owing to the elevation of a combustion temperature. The more the thermal NOx is produced, the higher is the concentration of oxygen, the higher is the combustion temperature, and the longer is the residence time of the exhaust gas at a high temperature.
Therefore, in order to decrease the amount of the formed thermal NOx, it is necessary to lower the oxygen concentration and the combustion temperature, and to rapidly complete the combustion for the sake of the curtailment of the combustion time.
On the basis of the above factors, there have been heretofore taken various measures to inhibit the formation of the NOx. These include the alternation of operating conditions, the decrease of excessive air, the reduction of heat generation in a combustion chamber, the drop of the preheating temperature of air for combustion, and the development of a low-NOx burner in which its structure is improved to control the production of NOx. However, when the above-mentioned measures other than the employment of the low-NOx burner are employed, an NOx value (assuming that O.sub.2 is 0%) in the exhaust gas is only lowered to about 100 ppm, which is still unsatisfactory.
The measure of employing the low-NOx burner can be applied to existing apparatus by a relatively simple reconstruction, and its NOx inhibition effect is large. For these reasons, the development of an effective low-NOx burner is desired.
However, the conventional NOx burner can decrease the NOx to some extent, but the employment of the conventional NOx burner lowers the combustion temperature, so that a combustion rate decreases and the flame stabilizing performance deteriorates, with the result that unstable combustion occurs. Furthermore, incomplete combustion occur due to the drop of the combustion temperature, so that carbon monoxide (CO) and soot are undesirably generated.
Japanese Utility Model Application Laid-open No. 2-140122 discloses one example of the conventional low-NOx gas burners. In this burner, fuel jet orifices and air orifices are arranged on the periphery of a nozzle so as to locally correspond to each other, and therefore when a combustion gas pressure is low (e.g., about 100 mmH.sub.2 O), air is mixed sufficiently with a fuel gas, so that the combustion temperature rises and the NOx concentration also undesirably increases.
In addition, according to data described in the same publication, an NOx value in the exhaust gas is from about 40 to 50 ppm, which should be further decreased.
The conventional low-NOx burner has the following drawbacks:
(1) When the conventional low-NOx burner is employed, the combustion temperature drops, and this fact leads to the decrease of the combustion rate, the deterioration of the flame stabilizing performance and the occurrence of the unstable combustion. PA1 (2) Additionally, the combustion temperature drops, and this fact leads to the incomplete combustion and the generation of carbon monoxide (CO) and soot. PA1 (3) When the fuel gas pressure is low (e.g., about 100 mmH.sub.2 O), air is mixed sufficiently with the fuel gas, so that the combustion temperature rises and the NOx concentration also increases. PA1 (4) The NOx value in the exhaust gas is from about 40 to 50 ppm, which is still at a high level. PA1 (1) a weak/strong combustion is carried out, PA1 (2) a flame dividing system is used, and PA1 (3) an exhaust gas self-recycling system is utilized. PA1 (1) the exhaust gas self-recycling system is utilized, and PA1 (2) a constitution to accelerate the mixing of a fuel gas with air is employed. The employment of a low-NOx burner having such a construction permits preventing the decrease of the combustion rate, the deterioration of the flame stabilizing performance and the occurrence of unstable combustion, even if the combustion temperature drops. In addition, irrespective of the drop of the combustion temperature, the low-NOx burner having the above-mentioned construction also prevents incomplete combustion, whereby the generation of carbon monoxide (CO) and soot can be inhibited. Moreover, even when a fuel gas pressure is low, the NOx concentration does not increase, and the NOx value in the exhaust gas can be controlled to a low level of about 30 to 40 ppm. As a consequence, the present invention has been now attained on the basis of the above-mentioned knowledge.